React.createElement: type should not be null with Meteor, TrackerReact, React-mounter and FlowRouter

I have this route,

FlowRouter.route('/dashboard',{
name: 'dashboard',
action(){
    mount(MainLayout, {
        content: (<AW />)
    })
}
});

I imported the AW file.

import React from 'react';
import TrackerReact from 'meteor/ultimatejs:tracker-react';
import UA from './UA.jsx';
export default class AW extends TrackerReact(React.Component){
    constructor(){
        super();
    }
    componentWIllUnmount() {
    }
render(){
    return (
    <div>
        <UA />
        </div>
    )
    }
}

If I removed the <UA /> it is working. If I placed <UA />, it is not working. The error is React.createElement: type should not be null,undefined,boolean, or number.

I tried some editing on the UA file. But unfortunately, nothing works.

Here is the UA file.

import React from 'react';

class UA extends React.Component{
render(){
    return(

        <div>hi</div>
    )
}
}
export default UA;